Two-phase brushless DC (BLDC) motors are widely used in fans for ventilating and cooling central processing units (CPUs), graphics processors, power supplies, and many other applications. An advantage of the BLDC motors is that they, compared to brushed DC motors, are lighter, accelerate faster, produce little electrical or acoustical noise, and require no maintenance (no brush wearing). The requirements for cooling fans are continuously increasing as the use of powerful cooling-requiring electronics are increasing. As more fans are installed in more products, the need for low-cost fan solutions is evident to keep the overall cost of end-products low. Further, as the number of fans in homes and offices is increased, the need to keep these fans as quiet and efficient as possible is also getting more pronounced. The characteristics of BLDC motors matches the requirements of fans very well.
BLDC motors comprise a rotor with permanent magnets driven by a stator with coils. The number of poles on the permanent magnets and the number of coils vary depending on the desired characteristics of the motor. FIGS. 1A and 1B illustrate a basic two-phase motor (phase 1 in FIG. 1A, phase 2 in FIG. 1B) with four magnetic rotor poles (two each of magnetic N and S) and four stator coils (SC1–SC4). Note that for fan motors, a typical arrangement has the stator physically located in the center and the rotor on the outside. The basic principal of operation is the same. The upper and lower coils (coils SC1 and SC2) are electrically connected to form a first set of coils, and the left and right coils (coils SC3 and SC4) are likewise electrically connected to form a second set of coils. In two-phase motors, the two sets of coils are not directly connected to each other at a center point, unlike three-phase motors.
A set of coils is active or energized, whenever a current flows through the active coils, causing a magnetic field to emerge that affects the rotor magnets. In two-phase motors, whenever one set of coils is active, the other set is passive. No drive current flows in the passive or unenergized coils to contribute a magnetic field that would affect the rotor magnets. The orientation of the rotor magnets relative to the stator coils determines when to energize a particular set of coils to get the desired motor rotation.
Applying current through coils SC1 and SC2 (energizing these coils) will push/pull the poles of the rotor magnets towards an alignment with the coils (Phase 1). Once rotating, the inertia of the rotor will ensure that the rotor is not only attracted to, but also passes, the active coils. As soon as the rotor magnets pass the active coils, the commutating must be changed so that the other set of coils (coils SC3 and SC4) are activated (Phase 2) and the rotation is thereby continued. If the commutation is made at the right moment, the magnetic fields generated by the coils are changed such that they never slow down the moving rotor. Alternatively, if the commutation were made too early or too late, a negative torque is produced for a short while, and thereby limiting the total torque and speed of the motor. It is therefore necessary to know the orientation of the rotor magnets relative to the coils in order to get maximum performance from the motor.
The commutation of two-phase BLDC motors is usually controlled by the use of Hall sensors to detect the orientation of the rotor. A Hall sensor is a magnetic switch responsive to the orientation of the magnetic field generated by the rotor magnets. The Hall sensor sets or clears its output dependent upon whether the rotor's north or south magnetic pole faces it. If the Hall sensor HS is placed halfway between coils SC2 and SC3, as seen in FIGS. 1A and 1B, then the Hall sensor HS changes its output when the rotor magnets align with the coils. Hall sensors allow a motor's control system to know when to commutate the stator coil (deactivating one set of coils and activating the other) in order to maintain the desired rotation. A down side to this rather simple arrangement is that the Hall sensor needs to be positioned accurately in order to provide reliable information to the motor controller. A Hall sensor is also a cost adder to an otherwise inexpensive motor.
A BLDC motor is highly influenced by its self-generated electromotive force (EMF). EMF is a voltage generated over an inductor (i.e., the stator coils) by a changing magnetic field (e.g., due to the turning of the rotor with its permanent magnetic poles). FIGS. 2A–2F and FIG. 3 illustrate the effect of the moving magnetic rotor poles N and S on the stator coil SC1. The resulting EMF voltage, shown in FIG. 3, has a waveform that depends upon the physical shape of the rotor magnets and coil ferro-core, seen here with a typical trapezoidal shape. The amplitude of the generated EMF depends upon the rate at which the magnetic field (as seen by the coil) changes, in this case by the speed of the motor. The EMF will, when the motor reaches a certain speed, be the same amplitude as, but in the opposite direction from, the drive voltage used to energize the coils. Because of this “Back EMF” (B-EMF) induced in the stator coils, there is a limit to the current flow through an energized coil, which is required to generate the magnetic field to cause rotor rotation. Hence there is a maximum speed of the motor for any given drive voltage.
For three-phase BLDC motors, commutation control methods based on sensorless feedback are common, where the motor's B-EMF is used to determine the rotor orientation. Physically, a three-phase motor uses a Y-configuration of three stator coils connected to a common center point, with two of the coils being active at any given moment during operation, and with a third coil not energized by a drive voltage and being considered as the passive coil for use as reference for measuring the generated B-EMF. Thus, in three-phase BLDC motors, one can measure the B-EMF in whichever coil is presently considered to be the passive coil relative to a center point between the three coils or relative to each other (the passive coil being effectively connected to a reference potential established between the two active coils). The result of this is that the moment of primary interest to commutating a three-phase motor is when the B-EMF changes polarity, i.e., the zero crossing of the signal.
It may be desired to control the speed of a motor. For example, in a cooling fan application a speed based on an external temperature would allow lower speeds (with corresponding reduction in power consumption and acoustical noise) whenever the temperature is relatively low, which could be increased to higher speeds as needed when the temperature becomes much higher. One way that motor speed might be controlled would be to adjust the operating voltage applied to the stator coils. However, very few systems, e.g., personal computers (PCs), can provide an adjustable operating voltage for their cooling fan motors. An additional complication of multi-speed motors is the response time needed to energize the stator coils. At high speeds, there is a phase lag between applying a drive voltage and obtaining the desired current flow through the coils. This phase lag may seriously affect the timing for commutation. At a single motor speed, the timing can be pre-adjusted to compensate. For multiple motor speeds, the adjustment becomes more complicated. For a simple PC cooling fan, one typically goes with a single-speed motor.